Interrupt-driven button handling in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when the button is pressed?

Consider this Arduino code that uses an interrupt to detect a button press. What will be printed to the serial monitor when the button is pressed?

Arduino

volatile bool buttonPressed = false;

void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), buttonISR, FALLING);
}

void loop() {
  if (buttonPressed) {
    Serial.println("Button pressed!");
    buttonPressed = false;
  }
}

void buttonISR() {
  buttonPressed = true;
}

A Button pressed! (printed continuously without stopping)

B Button pressed! (printed once per press)

C No output because interrupts are not enabled

D Compilation error due to ISR function

💡 Hint

Think about what happens inside the interrupt and how the main loop reacts.

The interrupt sets buttonPressed to true once per button press. The main loop detects this and prints the message once, then resets the flag. This prevents continuous printing.

🧠 Conceptual

intermediate

Why use volatile keyword for variables shared with ISR?

In interrupt-driven button handling, why is the volatile keyword used for variables shared between the ISR and the main code?

A To make the variable constant and unchangeable

B To allow the variable to be accessed only inside the ISR

C To tell the compiler the variable can change unexpectedly and prevent optimization errors

D To allocate the variable in EEPROM memory

💡 Hint

Think about how the compiler might optimize code if it assumes a variable never changes.

The volatile keyword tells the compiler the variable can change at any time, such as inside an ISR, so it must always read its current value from memory and not optimize it away.

🔧 Debug

advanced

Why does this interrupt code cause a crash?

Examine this Arduino code snippet. Why does it cause the program to crash or behave unpredictably?

Arduino

volatile int count = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), incrementCount, FALLING);
  Serial.begin(9600);
}

void loop() {
  Serial.println(count);
  delay(1000);
}

void incrementCount() {
  count++;
  delay(10);
}

A Calling delay() inside an ISR causes the crash

B The variable count is not declared volatile

C attachInterrupt is called with wrong pin number

D Serial.println() cannot be used in loop

💡 Hint

Think about what functions are safe to call inside an ISR.

Using delay() inside an ISR is unsafe because it relies on interrupts and timers that are disabled during ISR execution, causing the program to hang or crash.

📝 Syntax

advanced

Which option correctly attaches an interrupt to pin 3 on FALLING edge?

Choose the correct Arduino code line to attach an interrupt to pin 3 that triggers on the FALLING edge.

A attachInterrupt(digitalPinToInterrupt(3), isrFunction, FALLING);

B attachInterrupt(3, isrFunction, FALLING);

C attachInterrupt(3, isrFunction, FALLING_EDGE);

D attachInterrupt(digitalPinToInterrupt(3), isrFunction, FALL);

💡 Hint

Remember the correct way to convert a pin number to an interrupt number.

The function digitalPinToInterrupt(pin) converts a pin number to the correct interrupt number. The mode must be FALLING. Options B and C use wrong parameters, D uses an invalid mode.

🚀 Application

expert

How many times will the LED toggle after 5 button presses?

This Arduino code toggles an LED each time a button connected to pin 2 is pressed using an interrupt. How many times will the LED state change after 5 distinct button presses?

Arduino

volatile int pressCount = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  pinMode(13, OUTPUT);
  attachInterrupt(digitalPinToInterrupt(2), countPress, FALLING);
}

void loop() {
  static int lastCount = 0;
  if (pressCount != lastCount) {
    digitalWrite(13, !digitalRead(13));
    lastCount = pressCount;
  }
}

void countPress() {
  pressCount++;
}

A 0 times

B 10 times

C 1 time

D 5 times

💡 Hint

Each button press increments the count and triggers a toggle in the main loop.

Each press increments pressCount. The loop detects the change and toggles the LED once per press. So after 5 presses, the LED toggles 5 times.